In relating also to other ligands, supppose that the cis position facilitated a certain ionic interaction (such as hydrogen bonding), this would certainly add to the cis compounds as the major product.

Also, heat, if a heat barrier is overcome, the cis product can be the major.

There are some, perhaps many others, for instance if the cis product serves as a reactant for a subsequent product one can technically say that the cis product takes precedence since eventual dynamics of the situation-cis product is used up, more cis is made, la chatelier, trans product declines.

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If another ligand were bidentate then that could force the two phosphine ligands to be cis to on another, since bidentate ligands tend to orient their chelating atoms in a cis fashion.

I think that the more subtle reason that phosphines tend to for trans complexes is the so-called "trans effect" which results from the electronics of the donor atoms and the acceptor orbitals on the metal. You can probably find an better explanation than that in any inorganic textbook.